Induction furnace with two melting loops



Jan. 30, 1968 1. BECKIUS ET AL 3,366,726

INDUCTION FURNACE WITH TWO MEETING LOOPS Filed June 50, 1965 INVENTURS gVAR Bevin); BY erv t Fae mKssorr AT TORNEYS United States Patent Office 3,366,726 Patented Jan. 30, 1968 ABSTRACT OF THE DISCLOSURE An induction furnace of the submerged resistor type fed from a multi-phase network has a hearth and two melting loops connected with the hearth. The loops have a common branch and individual branches. The coils of the two branches are two-phase fed. One of the individual branches has a substantially greater cross-section than the other and the coil of such branch is fed from the phase subsequent to the phase which feeds the coil of the other individual branch.

The present invention relates to an induction furnace of the submerged resistor type for inductive melting, fed from a multi-phase, preferably three-phase network. The furnace is of the type which contains at least two melting loops, situated close to each other and provided with a common part.

In these furnaces the loops enclose their own primary coil with an iron core section, which coils are preferably fed with two-phase current from a multi-phase network, for example through a Scott connection or a V-connection. The Scott connection can be carried out on the furnace coils themselves or can also be carried out in a network transformer (see below). In a V-connection, the secondary side of the transformer is connected in a triangle with (for three-phase) one phase open and the furnace coils connected to the end points of the two remaining phases. In previously used induction furnaces with several melting loops, the loops have usually been made with equally large sectional areas at the side of the common part of the loops. The furnace coils are similarly formed. When a certain power is fed to the furnace, the load on both the coils, i.e. the powers fed to the two loops become essentially different. The loop which will receive the lowest power is dependent on the phase sequence of the core flux or the phase sequence of the feeding network in relation to the connection between the network phases and the furnace coils. The total power which is of main importance for efliciency becomes even lower when fed by a symmetrical voltage system than if the power division were more symmetrical.

The explanation of this asymmetrical condition is the influence of the common inductance between the circuits of the two loops. In FIG. 1 a Scott connection (threephase to two-phase) is shown. The two output voltages are shown vectorially E and E The resistance in the common part of the loop is designated R and (we assume for this reasoning the same area for the remaining parts of the two loops, i.e. free parts) R for the free parts. L is the corresponding leak flux of the self inductance of each circuit between the loop and the primary coil, while N is the mutual inductance between, on the one hand the one loop and the relevant primary coil and, on the other hand, the other loop and its relevant primary coil. Indexes A and B refer to their respective loops and M to the common part. A lies in phase sequence before B.

For the currents in the different loop parts the following expressions are valid:

L M 1 =1 (1+ iI I Nil I 1 I E and E are current and voltage vectors respectively. w==2-1r-f, where f is the frequency.

The denominator expression (N) is The voltage E and E of the Scott system are mutually phase displaced and have the same numerical value E. The expressions for the loads which belong to the transformer phases A and B are Active effects PA=E']A'COS (PA P =E-J 'cos (p Reactive effects QA= A' PA QB= B-Sin (PB From the equation system for the currents above, it is evident that the resistance in the common part R of the loops and the mutual inductance M cause the currents I and I to become unequally large, which means that even the powers P and P become unequally large. This difference is considerable.

The invention refers to a solution of these problems and is characterised in that the melting loops at the side of the common part are formed with mutually different cross sectional areas. Suitably the induction furnace is made according to the invention of a furnace with two induction coils, two-phase fed from a three-phase network, for example through a Scott connection, and the melting loop whose coil is fed with the subsequent phase is given greater area than the other melting loop.

The invention is further exemplified in the accompanying figures, of which FIG. 1 shows an induction furnace with two melting loops and the electric connection to these, while FIG. 2 shows a diagram for active power at different area ratios.

In FIG. 1, 11 is a common melting space and 12 and 13 are loop parts with different areas at the side of the common part 14. I and 1 are the currents in the parts 12 and 13. The loops for melting the contents of the furnace, such as steel, surround the primary coils 15 and 16 and their iron cores 17 and 18. The coils are fed in two-phase from the Scott connected transformer 19, whose primary side is three-phase fed (network frequency). E and E are the voltages fed from the coils 15 and 16 and in the loops 12 and 13 melting of the charge is produced the charge and also a certain circulation of the charge. The induction furnace can possibly be made to operate in vacuum. E lies in the phase before E The area of the loop 13 (at the side of the part 14) is greater than the area of loop 12. If the phase sequence should be the opposite, the loop 12 is made with a greater area than the loop 13.

At different cross sections, resistance R resistance R (R and R being the resistance for the free parts in A (12) and B (13) respectively) and E 90 response time relative to E is valid for the currents The self inductances L and L are also influenced by making the areas unequal in such a way that the power balance is counteracted. The change of these inductances with the cross sectional changes is thus considerably less than the resistance change, by means of which the counteraction becomes slight.

In FIG. 2 a diagram for the power is shown, the total power P and active powers in A and B, i.e. P and P with different area ratios k=a :a where a and al are the areas in the free loop parts. As seen, with the reduction'of k down to 0.5 (1:2) a considerable equalisation is produced, but at approximate k=0.75 the equalisation begins to increase. A lower limit for k should be fixed at 0.4 for constructional reasons and then also there will be a certain quantity of melt in the smaller loop. In other words, the ratio of the cross-sectional area of the smaller loop (a,,) to the cross-sectional area of the larger loop (61 is between 3:4 and 0.4:1. With k=0.5 the total active power has been increased by 24% and the effect factor (cos (p) has risen by 35%, which means that this step-like active effect is produced with a lower network loading.

The invention can also be applied in V-connection, in which case the angle difference between E and B is 60 or 120. The invention can also be varied in other ways within the scope of the following claims.

What is claimed is:

1. Induction furnace of the submerged resistor type, fed from a multiphase network, said furnace comprising a hearth and at least two melting loops beginning and ending in said hearth, said loops having a common branch and individual branches, one of said individual branches having a substantially greater cross-sectional area than the other individual branch.

2. Furnace in accordance with claim 1, having two induction coils, one for each loop, two-phase fed from a three-phase network, the individual branch of the loop having a coil fed from a subsequent phase having said greater cross-sectional area than the corresponding individual branch of the other loop.

3. Furnace in accordance with claim 2 in which the ratio of the cross-sectional area of the small loop (11 to the cross-sectional area of the larger loop (a is between 3:4 and 0.4: 1.

References Cited FOREIGN PATENTS 11/ 1941 Great Britain.

7/ 1953 Germany. 

